Formed articles of inorganic fibers typified by ceramic fibers have been used for applications exposed to high temperatures, for example, industrial heat insulators, refractory materials, and packing materials. In recent years, such formed articles have also been used as cushioning materials (catalyst-holding material) for automotive exhaust gas cleaning devices. That is, such formed articles have also been used as mats for exhaust gas cleaning, in which mats are wound around catalyst-supporting members and arranged between catalyst-supporting members and metal casings when catalyst-supporting members are accommodated in metal casings.
For example, in the case where such an inorganic fiber formed article is processed as a heat insulator or processed into an automotive catalyst-holding material (mat), needling treatment (needle punching treatment) is commonly performed in order to control the thickness and the surface density.
Furthermore, in order to prevent the scattering of fibers during an assembling operation, an organic binder is commonly incorporated into such a mat (Patent Document 1).
However, in a conventional inorganic fiber formed article, when the thickness is ensured, the surface smoothness is not ensured. Furthermore, for example, a phenomenon occurs in which a surface layer is peeled. This leads to problems, such as the occurrence of a partial density difference and the deterioration of workability, when the article is shaped as a heat insulator.
Moreover, an organic binder-containing mat has the following problems: The presence of the organic compound increases harmful gases due to exhaust heat generated during engine combustion. Furthermore, the mat surface pressure is temporarily reduced, i.e., the holding force is reduced, until the decomposition of the organic compound is completed.
With respect to a method for overcoming the problems, it is reported that a mat has a minimized organic binder content or is free from an organic binder. A mat that does not contain an organic binder is bulky, so it is difficult to reduce the thickness. Thus, when the mat is fitted into a metal casing, the mat can be delaminated. Furthermore, the absence of an organic binder disadvantageously causes the emission of a large amount of fibrous dust from the mat when the mat is wound around a catalyst-supporting member or fitted into the metal casing, thereby significantly deteriorating the workability.